Cement compositions with improved mechanical properties and methods of cementing in subterranean formations

ABSTRACT

Foamed cement compositions comprising carbon fibers, a hydraulic cement material, sufficient water to form a slurry, an expanding additive, and optionally, other ingredients, including any suitable additives.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional patent application of commonly-ownedU.S. patent application Ser. No. 10/435,297, filed May 9, 2003, entitled“Cement Compositions with Improved Mechanical Properties and Methods ofCementing in Subterranean Formations,” by Lance E. Brothers, et al.,which is incorporated by reference herein for all purposes.

BACKGROUND

The present invention relates to foamed cementing operations insubterranean zones, and more particularly, to foamed well cementcompositions having improved mechanical properties and methods of usingthe compositions in subterranean well cementing operations.

Hydraulic cement compositions are commonly utilized in subterraneanoperations, particularly subterranean well completion and remedialoperations. For example, hydraulic cement compositions are used inprimary cementing operations whereby pipe strings such as casings andliners are cemented in well bores. In performing primary cementing,hydraulic cement compositions are pumped into the annular space betweenthe walls of a well bore and the exterior surface of the pipe stringdisposed therein. The cement composition is permitted to set in theannular space, thereby forming an annular sheath of hardenedsubstantially impermeable cement therein that substantially supports andpositions the pipe string in the well bore and bonds the exteriorsurfaces of the pipe string to the walls of the well bore. Hydrauliccement compositions also are used in remedial cementing operations suchas plugging highly permeable zones or fractures in well bores, pluggingcracks in holes in pipe strings, and the like.

Cement compositions utilized in well applications often need to belightweight to prevent excessive hydrostatic pressure from being exertedon subterranean formations penetrated by the well bore whereby theformations are unintentionally fractured. Thus, foamed cementcompositions are often used in subterranean well applications. Inaddition to being lightweight, a foamed cement composition containscompressed gas which improves the ability of the composition to maintainpressure and prevent the flow of formation fluids into and through thecement composition during its transition time, i.e., the time duringwhich the cement composition changes from a true fluid to a hardenedmass. Foamed cement compositions are also advantageous, because theyhave low fluid loss properties. Additionally, foamed cements have alower modulus of elasticity than non-foamed cements, which is usuallydesirable as it enables the cement, inter alia, to resist hoop stresseswhen the cement encases pipe that expands from internal pressures.

A stable foamed cement may be generated in situ in circumstances such aswhen the cement composition contains an expanding additive, such as afine aluminum powder, which generates a gas within the composition as itreacts with the high pH of the cement slurry. In other cases, a stablefoamed cement may be generated when Portland cement, or any otherhydraulic cement, has air or a compressed gas such as nitrogen injectedwith proper surfactants.

Set cement in subterranean formations, and particularly the set cementsheath in the annulus of a well bore, may fail due to, inter alia, shearand compressional stresses exerted on the set cement. This may beparticularly problematic in hostile subterranean formations. In thesetypes of formations, set cements often fail as a result of the stressesexerted on the set cement.

The stress exerted on the cement as referred to herein means the forceapplied over an area resulting from the strain caused by the incrementalchange of a body's length or volume. The stress is generally thought tobe related to strain by a proportionality constant known as Young'sModulus. Young's Modulus is known to characterize the flexibility of amaterial.

There are several stressful conditions that have been associated withwell bore cement failures. One example of such a condition results fromthe relatively high fluid pressures and/or temperatures inside of theset casing during testing, perforation, fluid injection, or fluidproduction. If the pressure and/or temperature inside the pipe increase,the resultant internal pressure expands the pipe, both radially andlongitudinally. This expansion places stress on the cement surroundingthe casing causing it to crack, or the bond between the outside surfaceof the pipe and the cement sheath to fail in the form of, inter alia,loss of hydraulic seal. Another example of such a stressful condition iswhere the fluids trapped in a cement sheath thermally expand causinghigh pressures within the sheath itself. This condition often occurs asa result of high temperature differentials created during production orinjection of high temperature fluids through the well bore, e.g., wellssubjected to steam recovery processes or the production of hot formationfluids. Other stressful conditions that can lead to cement failuresinclude the forces exerted by shifts in the subterranean formationssurrounding the well bore or other over-burdened pressures.

Failure of cement within the well bore can result in radial orcircumferential cracking of the cement as well as a breakdown of thebonds between the cement and the pipe or between the cement sheath andthe surrounding subterranean formations. Such failures can result inlost production, environmental pollution, hazardous rig operations,and/or hazardous production operations. A common result is theundesirable presence of pressure at the well head in the form of trappedgas between casing strings. Additionally, cement failures can beparticularly problematic in multi-lateral wells, which include verticalor deviated (including horizontal) principal well bores having one ormore ancillary, laterally extending well bores connected thereto.

In both conventional single bore wells and multi-lateral wells havingseveral bores, the cement composition utilized for cementing casing orliners in the well bores must develop high bond strength after settingand also have sufficient resiliency, e.g., elasticity and ductility, toresist loss of pipe or formation bonding, cracking and/or shattering asa result of all of the stressful conditions that may plague the well,including impacts and/or shocks generated by drilling and other welloperations.

Because a typical foamed cement composition generally may have a lowertensile strength than typical non-foamed cement, a foamed cement can bemore susceptible to these stressful conditions. As a result, foamedcements may not be as useful in subterranean applications.

SUMMARY

The present invention relates to foamed cementing operations insubterranean zones, and more particularly, to foamed well cementcompositions having improved mechanical properties and methods of usingthe compositions in subterranean well cementing operations.

In one embodiment, a cement composition of the present inventioncomprises a hydraulic cement; carbon fibers; water present in an amountsufficient to form a cement slurry; and an expanding additive whereinthe expanding additive comprises at least one of the following:aluminum, gypsum, or magnesium oxide.

In another embodiment, a foamed cement composition of the presentinvention comprises a hydraulic cement; carbon fibers wherein at least aportion of the carbon fibers have a mean length less than about 1 mm;water present in an amount sufficient to form a cement slurry; and anexpanding additive present in an amount sufficient to foam thecomposition.

In another embodiment, a foamed cement composition of the presentinvention comprises a hydraulic cement; carbon fibers wherein the carbonfibers are present in an amount of from about 1% to about 15% by weightof the cement in the cement composition; water present in an amountsufficient to form a cement slurry; and an expanding additive present inan amount sufficient to foam the composition.

The features and advantages of the present invention will be apparent tothose skilled in the art. While numerous changes may be made by thoseskilled in the art, such changes are within the spirit of the invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The present invention relates to foamed cementing operations insubterranean zones, and more particularly, to foamed well cementcompositions having improved mechanical properties and methods of usingthe compositions in subterranean well cementing operations. While thecompositions and methods are useful in a variety of subterraneanapplications, they are particularly useful for subterranean wellcompletion and remedial operations, such as primary cementing, e.g.,cementing casings and liners in well bores, including those inmulti-lateral subterranean wells.

The improved cement compositions of the present invention comprise acement composition that further comprises a hydraulic cement, carbonfibers, water sufficient to form a pumpable slurry, and an expandingadditive capable of causing a gas to become incorporated within thecement composition. This incorporation of a gas into the cementcomposition is referred to herein as “foaming” the cement composition,which results in a “foamed cement.” The expanding additive may be a gasor any other additive, such as a particulate additive, that leads to theincorporation of a gas within the composition at a desired point in theprocess. Other additives suitable for use in subterranean well borecementing operations also may be added to these compositions if desired.

Any cement suitable for use in subterranean applications are suitablefor use in the present invention. Preferably, in one embodiment, theimproved cement compositions of the present invention comprise ahydraulic cement. A variety of hydraulic cements are suitable for useincluding those comprised of calcium, aluminum, silicon, oxygen, and/orsulfur, which set and harden by reaction with water. Such hydrauliccements include, but are not limited to, Portland cements, pozzolanacements, gypsum cements, high alumina content cements, silica cements,and high alkalinity cements. One example of a cement is commerciallyavailable under the trade designation “THERMALOCK” available fromHalliburton Energy Services in Duncan, Okla., which is a calciumphosphate cement, described further in U.S. Pat. No. 6,488,763, which isassigned to the assignee of the present application and is incorporatedherein by reference. Preferably, however, where the expanding additiveis a particulate, the most suitable cements are Portland cements or anyother cements which have a suitably high pH, preferably above 12. Wherethe expanding additive is a gas, any cement suitable for use insubterranean well cementing operations may be used.

The water utilized in the cement compositions of this invention can befresh water, salt water (e.g., water containing one or more saltsdissolved therein), brine (e.g., saturated salt water produced fromsubterranean formations), or seawater. Generally, the water can be fromany source provided that it does not contain an excess of compounds thatadversely affect other components in the cement composition. The watermay be present in an amount sufficient to form a pumpable slurry. Moreparticularly, the water is present in the cement compositions in anamount in the range of from about 25% to about 100% by weight of cementtherein, more preferably in the range of from about 30% to about 50% byweight of cement material therein.

The carbon fibers that are present in the cement compositions of thepresent invention are preferably high tensile modulus carbon fibers,which most preferably have a high tensile strength. In certain preferredembodiments, to achieve certain of the advantages associated with thepresent invention, the tensile modulus of the fibers exceeds 180 GPa,and the tensile strength of the fibers may exceed 3000 MPa. The fiberspreferably have a mean length of about 1 mm or less. In certainpreferred embodiments, the mean length of the carbon fibers is fromabout 50 to about 500 microns. Most preferably, the fibers have a meanlength in the range of from about 100 to about 200 microns. Preferably,they are milled carbon fibers. An example of suitable carbon fibersincludes “AGM-94” carbon fibers commercially available from AsburyGraphite Mills, Inc., of Asbury, N.J. AGM-94 fibers have a mean lengthof about 150 microns and a diameter of about 7.2 microns. Anotherexample of suitable carbon fibers includes the “AGM-99” carbon fibers,also available from Asbury Graphite Mills, Inc., which have a meanlength of about 150 microns and a diameter of about 7.4 microns.Preferably, the carbon fibers are present in the amount of from about 1%by weight of cement to about 15% by weight of cement in the cementcomposition.

The expanding additive may be any component suitable for incorporatinggas into the cement composition. Further, foaming of the cementcomposition can be accomplished by any suitable method. In one preferredembodiment, the cement is foamed by direct injection of the expandingadditive into the composition. For instance, where the cementcomposition is foamed by the direct injection of gas into thecomposition, the gas utilized can be air or any suitable inert gas, suchas nitrogen, or even a mixture of such gases. Preferably, nitrogen isused. Where foaming is achieved by direct injection of gas, the gas ispresent in the composition in an amount sufficient to foam thecomposition, generally in an amount in the range of from about 0.01% toabout 60% by volume of the composition. In another preferred embodiment,the cement is foamed by gas generated by a reaction between the cementslurry and an expanding additive present in the cement in particulateform. For example, the composition may be foamed by hydrogen gasgenerated in situ as the product of a reaction between the high pHslurry and fine aluminum powder present in the cement. Where anexpanding additive in particulate form is used, aluminum powder, gypsumblends, and deadburned magnesium oxide are preferred. Preferredexpanding additives comprising aluminum powder are commerciallyavailable under the tradenames “GAS-CHEK®” and “SUPER CBL” fromHalliburton Energy Services of Duncan, Okla.; a preferred expandingadditive comprising a blend containing gypsum is commercially availableunder the tradename “MICROBOND” from Halliburton Energy Services ofDuncan, Okla.; and preferred expanding additives comprising deadburnedmagnesium oxide are commercially available under the tradenames“MICROBOND M” and “MICROBOND HT” from Halliburton Energy Services ofDuncan, Okla. Such preferred expanding additives are described in U.S.Pat. Nos. 4,304,298, 4,340,427, 4,367,093, 4,450,010, and 4,565,578,which are assigned to the assignee of the present application and areincorporated herein by reference.

Where the expanding additive is a gas, foaming of the cement compositionis preferably achieved at the surface, and the foamed cement compositionis then introduced into the subterranean formation and permitted to settherein into a high strength, resilient, ductile, and tough foamedcement mass.

It has been found that foaming a cement composition affects themechanical properties of the cement composition by, inter alia, reducingits density and improving its elasticity. This may be desirable forcertain reasons. However, when a cement composition is foamedsufficiently to affect desirably the elasticity of the cement, thetensile strength of the cement may be adversely affected. The risk ofrupture of the cement sheath in response to a stressful condition isdirectly linked to the tensile strength of the cement, and the risk isattenuated when the ratio of the tensile strength of the cement to itsYoung's Modulus is increased. Thus, increasing the tensile strength ofthe cement by adding carbon fibers is desirable to increase the tensilestrength of the foamed cement composition. Also, adding carbon fibers asopposed to other additives, such as polypropylene, has the added benefitof providing increased temperature stability to the cement composition.This makes the cement compositions of the present invention especiallysuitable for use in or in conjunction with hostile subterraneanconditions, such as high temperatures and/or high pressures.

As will be recognized by those skilled in the art, when the cementcompositions of the present invention are utilized for primary orremedial subterranean well operations, such compositions can alsoinclude additional suitable additives, for example, fluid loss agents,weighting materials, and the like. The foamed cement compositions of thepresent invention can also include other additives such as accelerantsor retarders, if desired. If an accelerant is used, the accelerant ispreferably calcium chloride and is present in an amount in the rangefrom about 1.0% to about 2.0% by weight of the cement in thecompositions. Fluid loss additives such as hydroxyethylcellulose,carboxymethylcellulose, carboxymethylhydroxyethylcellulose,hydroxypropylcellulose, hydroxypropylguar, guar, polyvinylalcohol, orpolyvinylacetate are also suitable. Where the cement composition isfoamed by the direct injection of a gas or mixture of gases, asurfactant may also be present in the cement composition. Anycommercially available surfactant may be used. An example is “ZONESEAL2000™,” commercially available from Halliburton Energy Services, Inc.,which is described in U.S. Pat. No. 6,063,738, which is assigned to theassignee of the present application and is incorporated herein byreference.

A preferred method of the present invention comprises providing a cementcomposition that comprises carbon fibers; injecting sufficient gas intothe composition to foam it to a chosen density; introducing this foamedcement composition to a subterranean well bore; and allowing the foamedcement composition to set therein. An example of a preferred cementcomposition prepared by this method is a composition of Class A Portlandcement, sufficient water to form a pumpable slurry, sufficient gas tofoam the composition to a density of 12 lb/gallon, 2% ZONESEAL 2000™surfactant by weight of water, and 5% milled carbon fibers having a meanlength of 150 microns by weight of the cement in the composition.

Another preferred method of the present invention comprises providing acement composition that comprises carbon fibers, water, and an expandingadditive in particulate form; introducing this cement composition into asubterranean well bore; evolving gas within the cement composition priorto developing substantial compressive strength; and then permitting thecomposition to set therein. An example of a composition prepared by thismethod is a composition comprising Class A Portland cement, 46% water byweight of the cement, 1% SUPER CBL expanding additive by weight ofcement, and 10% milled carbon fibers having a mean length of 150 micronsby weight of the cement in the composition.

To facilitate a better understanding of the present invention, thefollowing examples of certain aspects of some embodiments are given. Inno way should the following examples be read to limit, or define, thescope of the invention.

EXAMPLE 1

Test samples of preferred embodiments of the cement compositions of thepresent invention were made, and the tensile strength of eachcomposition was determined. Comparative samples were also made andsimilarly tested. The test foamed cement compositions depicted inExample 1 were prepared by mixing Class A Portland Cement with 46% byweight of the cement water and foamed with air to a density of 12lb/gallon. ZONESEAL 2000™ surfactant was added to the foamed cement inan amount equal to 2% by weight of water, and the composition was curedfor 24 hours at ambient temperature. To certain sample cementcompositions, carbon fibers were added in chosen ratios as described inTable 1. The carbon fibers were milled fibers, specifically AGM-99fibers from Asbury Graphite Mills Inc., with a mean length of 150microns and a diameter of 7.4 microns. The tensile strength of eachcement composition was then determined in accordance with ASTM C496-96.

Table 1 below lists the percentage of carbon fibers that were added toeach cement composition and the resultant tensile strength. TABLE 1Milled Carbon Fibers Tensile Sample Water-to- (% by weight of StrengthDescription Cement Ratio cement) (psi) Comparative 0.46 0 115 Sample No.1 Comparative 0.46 5 160 Sample No. 2

Comparative Sample No. 1 illustrates the tensile strength of a foamedcement composition when no carbon fibers have been added to thecomposition. The tensile strength was 115 psi.

Comparative Sample No. 2 illustrates the tensile strength of a foamedcement composition containing carbon fibers. The tensile strength was160 psi, a 39% increase from Comparative Sample No. 1.

EXAMPLE 2

The test foamed cement compositions depicted in Example 2 were preparedby mixing Class A Portland Cement with 46% water by weight of the cementand 1% SUPER CBL expanding additive by weight of the cement. Thecomposition was cured for 24 hours at 150° F. To certain sample cementcompositions, carbon fibers were added in chosen ratios as described inTable 2. The carbon fibers were milled fibers, specifically AGM-94fibers from Asbury Graphite Mills Inc., with a mean length of 150microns and a diameter of 7.2 microns. The tensile strength of eachcement composition was then determined in accordance with ASTM C496-96.

Table 2 below lists the percentage of carbon fibers that were added toeach cement composition and the resultant tensile strength. TABLE 2Milled Carbon Fibers Tensile Sample Water-to- (% by weight of StrengthDescription Cement Ratio cement) (psi) Comparative 0.46 0 258 Sample No.3 Comparative 0.46 10 418 Sample No. 4

Comparative Sample No. 3 illustrates the tensile strength of a foamedcement composition when no carbon fibers have been added to thecomposition. The tensile strength was 258 psi.

Comparative Sample No. 4 illustrates the tensile strength of a foamedcement composition containing carbon fibers. The tensile strength was418 psi, a 62% increase from Comparative Sample No. 3.

EXAMPLE 3

It has been noted that lower water-to-cement ratios may affect realizedtensile strength. Sample cement compositions were prepared by mixingClass A Portland Cement with 38% water by weight of the cement andfoamed with air to a density of 12 lb/gallon. ZONESEAL 2000™ surfactantwas added to the foamed cement in an amount equal to about 2% by weightof water, and the composition was cured for 24 hours at 90° F. Carbonfibers were added to one sample composition in an amount equal to 5% byweight of the cement. The carbon fibers were milled fibers, specificallyAGM-99 fibers from Asbury Graphite Mills Inc., with a mean length of 150microns and a diameter of 7.4 microns. The tensile strength of eachcement composition was then determined in accordance with ASTM C496-96.The sample containing carbon fibers demonstrated a 6.0% increase intensile strength as compared to the sample which lacked carbon fibers.

Therefore, the present invention is well adapted to attain the ends andadvantages mentioned as well as those that are inherent therein. Whilenumerous changes may be made by those skilled in the art, such changesare encompassed within the spirit of this invention as defined by theappended claims. The terms in the claims have their plain, ordinarymeaning unless otherwise explicitly and clearly defined by the patentee.

1. A cement composition comprising: a hydraulic cement; carbon fibers;water present in an amount sufficient to form a cement slurry; and anexpanding additive wherein the expanding additive comprises at least oneof the following: aluminum, gypsum, or magnesium oxide.
 2. The cementcomposition of claim 1 wherein the expanding additive further comprisesat least one of the following: a gas, air, or nitrogen.
 3. The cementcomposition of claim 1 wherein the expanding additive is present in anamount in the range of from about 0.01% to about 60% by volume of thecement composition.
 4. The cement composition of claim 1 wherein thecarbon fibers are present in an amount of from about 1% to about 15% byweight of the cement in the cement composition.
 5. The cementcomposition of claim 1 wherein the hydraulic cement comprises at leastone of the following: a Portland cement, a pozzolana cement, a gypsumcement, a high alumina content cement, a silica cement, a highalkalinity cement, and a calcium phosphate cement.
 6. The cementcomposition of claim 1 further comprising a surfactant, a fluid lossagent, a weighting agent, an accelerant, or a retardant.
 7. The cementcomposition of claim 1 wherein the expanding additive is a powder. 8.The cement composition of claim 7 wherein the hydraulic cement comprisesa cement having a pH greater than about
 12. 9. The cement composition ofclaim 1 wherein at least a portion of the carbon fibers have a meanlength of about 100 to about 200 microns.
 10. The cement composition ofclaim 1 wherein the cement composition has a tensile strength greaterthan about 115 psi.
 11. The cement composition of claim 7 wherein thecement composition has a tensile strength greater than about 258 psi.12. The cement composition of claim 1 wherein the water is present in anamount in the range of from about 25% to about 100% by weight of cement.13. The cement composition of claim 1 wherein the water is present in anamount in the range of from about 30% to about 50% by weight of cement.14. A cement composition comprising: a hydraulic cement; carbon fiberswherein at least a portion of the carbon fibers have a mean length lessthan about 1 mm; water present in an amount sufficient to form a cementslurry; and an expanding additive present in an amount sufficient tofoam the cement composition.
 15. The cement composition of claim 14wherein the expanding additive is a powder and wherein the hydrauliccement comprises a cement having a pH greater than about
 12. 16. Thecement composition of claim 14 wherein at least a portion of the carbonfibers have a mean length of about 100 to about 200 microns.
 17. Acement composition comprising: a hydraulic cement; carbon fibers whereinthe carbon fibers are present in an amount of from about 1% to about 15%by weight of the cement in the cement composition; water present in anamount sufficient to form a cement slurry; and an expanding additivepresent in an amount sufficient to foam the cement composition.
 18. Thecement composition of claim 17 wherein wherein the expanding additive isa powder and wherein the hydraulic cement comprises a cement having a pHgreater than about
 12. 19. The cement composition of claim 17 wherein atleast a portion of the carbon fibers have a mean length of about 100 toabout 200 microns.
 20. The cement composition of claim 17 wherein atleast a portion of the carbon fibers have a mean length less than about1 mm.